1. Field of the Invention
This invention relates to systems for manipulating electrical loads, and more particularly to systems for defrosting refrigerators.
2. Description of Related Art
Conventional refrigeration systems require defrosting, that is, a removal of layers of ice or frost which build up on the interior of a cooled compartment. Defrosting refrigeration systems requires large amounts of energy. It would be desirable for a supplier of electricity to have the ability to control precisely when a customer's refrigeration system is defrosted; it is preferable for defrosting to occur during low demand periods, so as to reduce the likelihood of a brownout, for example.
Conventional refrigeration systems utilize a simple timing mechanism to regulate when the defrost cycle is to occur, usually triggering the defrost cycle merely at regular intervals. The drawback to such a system is that the time of day is not considered when the defrost cycle is triggered. As such, the defrost cycle may be commenced during high demand periods, to the inconvenience and detriment of the electricity supplier.
Some devices have been employed to reduce the amount of energy used during defrosting, which somewhat reduces the load that the defrosting refrigerator places on the electricity supplier. Other simple timing devices have been employed to start the defrost cycle at a specific time of day. These devices are limited, however, in that atypical spikes in electricity demand create unpredictable high demand periods for which a simple timing device cannot account.
Moreover, the predetermined time at which the defrost cycle is triggered is typically set either by the end user or by a microprocessor in the end user's refrigerator, and not by the electricity supplier. The average end user or such microprocessor is unaware of or indifferent to the load patterns of the electricity supplier; thus it is doubtful that an end user can make a useful determination as to what time of day his refrigeration system should be defrosted. Further, in these types of conventional devices, the electricity supplier is offered no control over the defrosting cycles of its end users.
Some of these prior art devices are described in U.S. Pat. Nos. 5,533,360 and 5,369,962 to Szynal et al., the teachings of which are incorporated herein by reference. Szynal et al. teach three common ways for an internal defrost cycle controller to monitor the refrigerator in which it is disposed: real time, cumulative time, and variable time methods. One embodiment of the Szynal et al. device includes a defrost cycle controller having a circuit with a plug-in module that can be used either as a variable time controller, a real time controller, or a cumulative run controller. In operation, a microprocessor senses signals which inform the microprocessor about the actual running of the compressor and the defrost heater. The microprocessor can determine the cumulative and continuous run times of the compressor and defrost heater and can thereby determine how to alter the operation of either or both to obtain increased efficiency.
The Szynal et al. devices suffer from the same drawbacks as those listed above. That is, they may be suitable to reducing the total amount of energy that a refrigerator might use, however they do not take into account high peak periods, nor do they allow for input from the electricity supplier regarding when the defrost cycle should and should not be activated.
U.S. Pat. No. 5,415,005 to Sterber et al. (which is herein incorporated by reference) purports to provide a device for initiating a defrost cycle during an off-peak demand period. Sterber et al. teach a control device whereby the initiation of the defrost cycle is responsive to the daily power consumption of the refrigerator. It includes a microprocessor which is programmed to determine the time of day without the usage of a clock by analyzing the energy consumption of the refrigerator during a 24-hour period. By determining the approximate time of day, the Sterber et al. control device is able to initiate a defrost cycle during what it believes to be an off-peak demand period.
The Sterber et al. device also suffers from the above-mentioned drawbacks of the prior art. Specifically, the Sterber et al. device does not actually allow the electricity supplier itself to have any input as to when the refrigerator defrost cycle should occur; instead, the microprocessor "guesses" as to what is an off-peak demand period based on the usage of the refrigerator. The microprocessor could easily be misled if it merely relies upon the one refrigerator's data. For example, in a household where all of the inhabitants go to work or to school, the refrigerator is typically fairly inactive between the hours of 9:00 am and 4:00 pm. The Sterber et al. control device might believe, because of this inactivity, that the ideal time to defrost the refrigerator is 1:00 pm. However, 1:00 pm is a point in the typical day when most businesses are operating at peak load; as a result, although the domestic refrigerator might not be at its most active, the time chosen represents a high-demand period from the electrical supplier's point of view.